1. Field of the Invention
The present invention is related to the control of the apparent compliance of a friction braking system. This system permits brake pedal characteristics, such as pedal effort and travel, experienced by the operator of a vehicle when achieving a desired braking result to be maintained at a relatively constant level, notwithstanding operation with varying amounts of powertrain braking.
2. Disclosure Information
Hybrid vehicles advantageously use regenerative braking to achieve enhanced fuel economy. In general, regenerative braking employs the use of an electric or hydraulic traction motor which functions not only as a motor, but also as a braking device, to power roadwheels attached to one or more axles of a vehicle. When the driver of the vehicle calls for braking, either by lifting off the accelerator, or by depressing the brake pedal, the vehicle may be decelerated entirely with regenerative braking at least initially, or with a combination of friction braking and regenerative braking, or with friction braking alone. Unfortunately, some hybrid vehicle braking systems produce a different response characteristic for the brake pedal when the vehicle is being operated with regenerative braking or the type of powertrain braking such as compression braking, as opposed to friction braking. This differential pedal feel arises because during intervals in which regenerative braking, or other powertrain braking, is being employed, it is not necessary for the vehicle's operator to depress the brake pedal with as much force to achieve a given deceleration, as would otherwise be the case where the vehicle is being braked by friction brakes alone.
A system according to the present invention maintains a relatively constant pedal feel across the full range of braking operation, whether operation is at 100% friction braking, or predominantly powertrain braking, such as regenerative braking.